A well-known mass-analyzing method for identifying a substance having a large molecular weight and for analyzing its structure is an MS/MS analysis (or tandem analysis). FIG. 11 is a schematic configuration diagram of a general MS/MS mass spectrometer disclosed in Patent Documents 1 and other documents.
In this MS/MS mass spectrometer, three-stage quadrupole electrodes 12, 13, and 15 each composed of four rod electrodes are provided, inside the analysis chamber 10 which is vacuum-evacuated, between an ion source 11 for ionizing a sample to be analyzed and a detector 16 for detecting an ion and providing a detection signal in accordance with the amount of ions. A voltage ±(U1+V1·cos ωt) is applied to the first-stage quadrupole electrodes 12, in which a direct current U1 and a radio-frequency voltage V1·cos ωt are synthesized. Due to the action of the electric field generated by this application, only a target ion having a specific mass-to-charge ratio m/z is selected as a precursor ion from among a variety of ions generated in the ion source 11 and passes through the first-stage quadrupole electrodes 12.
The second-stage quadrupole electrodes 13 are placed in the well-sealed collision cell 14, and Ar gas for example as a CID gas is introduced into the collision cell 14. The precursor ion sent into the second-stage quadrupole electrodes 13 from the first-stage quadrupole electrodes 12 collides with Ar gas inside the collision cell 14 and is dissociated by the collision-induced dissociation to produce a product ion. Since this dissociation has a variety of modes, two or more kinds of product ions with different mass-to-charge ratios are generally produced from one kind of precursor ion, and these product ions exit from the collision cell 14 and are introduced into the third-stage quadrupole electrodes 15. Since not every precursor ion is dissociated, some non-dissociated precursor ions may be directly sent into the third-stage quadrupole electrodes 15.
To the third-stage quadrupole electrodes 15, a voltage ±(U3+V3·cos ωt) is applied in which a direct current U3 and a radio-frequency voltage V3·cos ωt are synthesized. Due to the action of the electric field generated by this application, only a product ion having a specific mass-to-charge ratio is selected, passes through the third-stage quadrupole electrodes 15, and reaches the detector 16. The direct current voltage U3 and radio-frequency voltage V3·cos ωt which are applied to the third-stage quadrupole electrodes 15 are appropriately changed, so that the mass-to-charge ratio of an ion capable of passing the third-stage quadrupole electrodes 15 is scanned to obtain the mass spectrum of the product ions generated by the dissociation of the target ion.
In a general MS/MS mass spectrometer, the length of the collision cell 14 in the direction along the ion optical axis C which is the central axis of the ion stream is approximately 150 through 200 mm. The gas pressure in the collision cell 14 is a few mTorr and higher than that of the analysis chamber 10 surrounding the collision cell 14. When an ion proceeds in a radio-frequency electric field in the atmosphere of comparatively high gas pressure, the kinetic energy of the ion attenuates due to a collision with the gas, so that the ion decelerates. In an extreme case, a decelerated ion could stop in the radio-frequency electric field.
In the case where an MS/MS mass spectrometer as previously described is used as a detector of a chromatograph such as a liquid chromatograph for example, it is necessary to repeatedly perform an analysis at predetermined intervals of time. Hence, if the ion's time delay is significant due to the speed reduction, an ion which should normally pass through the third-stage quadrupole electrodes 15 might not be able to pass through it, which causes a degradation in the detection sensitivity. In addition, an ion remaining in the collision cell 14 may appear at a timing at which no ion should appear in reality, which causes a ghost peak.
Moreover, since it takes time for an ion to reach the detector 16, the time interval of the repeated analysis is required to be previously determined in view of such a situation, which might cause an omission of analysis information in a multi-component analysis.
One conventional and general method for avoiding the previously described various problems is to form a direct current electric field having a potential gradient in the ion's passage direction in the collision cell 14, so that an ion should be accelerated by the action of the direct current electric field. In the mass spectrometer described in Patent Document 2, an electric field having a potential gradient in the direction of the ion optical axis is formed in order to accelerate an ion by the application of a direct current voltage to a radio-frequency ion guide in which each rod electrode has a different tilt to the ion optical axis, or by the application of a direct current voltage to each rod divided in the direction of the ion optical axis. In the mass spectrometer described in Patent Document 3, an ion that is allowed to pass through is accelerated by sequentially applying a pulse voltage to each aperture electrode of a radio-frequency ion guide in which approximately one hundred aperture plates are arranged in the direction of the ion optical axis.
However, if each rod electrode of a radio-frequency ion guide is obliquely disposed at different angles from each other or if an auxiliary electrode is used in order to form a direct current electric field having a potential gradient in the direction of the ion optical axis, a turbulence might occur in the radio frequency electric field appropriate for converging ions, which might deteriorate the ion passing properties. In addition, the configuration of Patent Document 3 has a complex structure, and simultaneously requires a complicated control since a pulse voltage for accelerating an ion should be appropriately controlled in accordance with each mass-to-charge ratio.
In the case where an atmospheric pressure ionization interface is used as in a liquid chromatograph mass spectrometer, a multi-stage differential evacuation system is used for maintaining a high vacuum atmosphere within an analysis chamber, which includes a mass separator and detector. In this case, the gas pressure inside the intermediate vacuum chamber in the subsequent stage of an ionization chamber is relatively high due do the atmosphere flowing from the ionization chamber, which causes the same problem as inside the collision cell as described earlier.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. H07-201304    [Patent Document 2] U.S. Pat. No. 5,847,386    [Patent Document 3] U.S. Pat. No. 6,812,453